So who has worked in the field with a mirrorless body. Try doing very early morning and very late afternoon landscape work and see how pleased you'll be. The optical viewfinder lcd is the weak link here. Great idea but at the moment a "nitch" market at best.
When the MKIII came out and everybody started going right I went left and bought NEX-7. I'm hooked. I use it as as a light weight second camera opposite my 7D and for video. I just used it last weekend as a bonus camera to video a wedding ceremony with an old 135mm Tamron Adaptall on it...the footage is great. I'll be using it in a couple of weeks to shoot a bunch of sporting events over the course of two weeks and I expect the compact size and 10fps to give my 7D a run for it's money...especially since my old 400mm f/5.6 AI lens fits on the NEX and the 7D has nothing in my bag that comes close unless I rent.
For guys like me, a mirrorless needs to be prosumer or better. APS-C is a must, 10fps is a must, active EF lens compatibility (an adapter is fine) is a must (especially since NEX already has this with a Metabones adapter). Video has to be better than the 7D to compete. High ISO noise has to be better than the 7D to compete.
If they can pull a camera out which is better than the NEX-7 (which already is overall a better camera than my 7D in my view), I'll preorder no matter the cost.
Full frame is neat but I think unrealistic....there'd be too many compromises and I'm not sure there's enough demand at the price they'd have to charge to keep it from competing with the MKIII on price. If it were the size of a GH2, mirrorless, full frame with MKIII capability, I might buy but I'm not convinced that's what this will be....it just doesn't make sense for Canon to do it business wise. Despite the capability of the GH2, that format of camera has never been popular.